1. Field of the Invention
The present invention relates to a high-frequency oscillator (HFO) ventilator.
2. Description of the Prior Art
An HFO ventilator operates to fully ventilate a patient by introducing pressure oscillations to a column of gas which is in communication with a patient""s airways. These oscillations cause the supply of breathing gas to and the active extraction of the supplied volume of gas from the airways of the patient in alternation. The peak-to-peak pressure amplitude about a mean airway pressure is typically between 0.05 and 0.2 bar and oscillates at a typical frequency of between 10 Hz and 50 Hz to supply a tidal volume significantly less than that required during spontaneous breathing, typically at or around anatomical dead-space volumes, and is usually less than that typically supplied by a jet device during HFJ ventilation.
This is also in marked contrast to the operation of conventional mechanical ventilators. Generally, a conventional mechanical ventilator operates to fully ventilate a patient by supplying breathing gas to the patient""s airways in an amount and at a frequency substantially equal to those of a spontaneously breathing patient. Typically then, for an adult, a conventional mechanical ventilator will provide a tidal volume of around 500 milliliters at a frequency of around 0.2 Hz.
An HFO ventilator generally has a gas conduit having an opening at one end for connection to the patient""s airways and an opposite end in gaseous communication with an oscillator. The oscillator typically includes a reciprocally movable element, such as a membrane or a piston, as part of a variable gas holding volume to which the end of the conduit is in gaseous communication. A drive unit is provided to reciprocate the movable element at a predetermined high-frequency to alternately remove a volume of gas from and return it to the gas conduit. Alternating under-and over-pressure pulses are thereby supplied to gas within the conduit at that frequency and travel along a respiration flow-path which connects the variable gas holding volume to the patient""s airways. This causes a column of gas, the volume of which is dependent on the volume change of the oscillator, to be moved out of and in to the patient""s airways and thereby provide ventilation. A continuous so called xe2x80x98biasxe2x80x99 flow of fresh breathing gas moves between an inlet and an outlet, along a flow path which intersects the respiration flow-path of the moving column of gas within the conduit. This bias flow washes carbon dioxide (CO2) rich gas, that has been drawn from the patient""s lungs by the under-pressure pulse, away from the respiration flow-path. The bias flow also maintains a mean positive airway pressure (or bias) about which pressure the high-frequency pressure pulses oscillate.
In an attempt to ensure adequate CO2 removal a typical bias flow of between 20 to 90 liters per minute, depending largely on whether a child or an adult is being ventilated, is employed. With this highest flow of around 90 liters per minute the CO2 elimination has proven inadequate for adults. Even with children the oscillating frequency has to be reduced so as to enable sufficient CO2 elimination (a reduction in oscillating frequency permits larger tidal volumes to be delivered and removed). However, the operation of the ventilator at less than the optimal physiological frequency often requires the delivered tidal volume to be increased to allow for a sufficient oxygen uptake by the patient, leading to a necessarily lower oscillating frequency, and an increase in bias flow rate.
It is an object of the present invention to provide a high-frequency oscillator ventilator wherein the aforementioned problems associated with known ventilators of this type are avoided, or are at least alleviated.
This object is achieved by the present invention wherein a high-frequency oscillator ventilator as generally described above is provided with an extraction device, such as a vacuum pump or a variable volume container, to remove from the respiration flow path an amount of CO2 rich breathing gas in addition to that removed by the bias flow, so as to reduce the amount of CO2 re-breathed by the patient.
Preferably, the extraction device is couplable in gaseous communication to the respiration flow-path at a location between the bias-gas flow path and a patient""s airways. This has the advantage that CO2 rich gas that would not be washed out with the bias flow and thus which would otherwise be re-breathed is removed from the system to be replaced by fresh gas from the bias gas flow.
The extraction device is operated in timed relationship with the oscillator to withdraw gas only during a part of the breathing cycle which predominantly comprises the expiration phase, and especially an end portion of the expiration phase, when the extracted gas may be analyzed to provide information, for example about end-tidal CO2 levels, useful in monitoring the efficacy of the HFO treatment. By timing the operation of the extraction device to be substantially within the expiration phase, the tidal volume delivered to the patient during the inspiration phase of the oscillator is substantially unaffected by the operation of the extraction device. A flow controller, such as a pressure regulator, is also provided in gas communication with the bias gas flow inlet to vary the flow of bias gas therethrough to compensate for the amount of gas removed by the periodic operation of the extraction device.